Always turn shoulders
One-handed backhand grip is found by pointing racket straight forward, and turning it to point left, whilst relaxing the right hand so that it ends up flat on the grip, with the top part of the hand perpendicular to the racket head.
In all strokes the plan is to lift the ball up and a long way over the net. The net is the enemy here.
This results in topspin, which brings it down again.
Therefore bring stroke from a long way back and finish with it over the shoulder. Intersecting with the ball is a secondary consideration.
For forehand use off-arm as counterweight, moving it in opposite direction to racket and other arm. Helps balance.
For backhand initially have hand on racket throat, far behind you. After initial racket movement release with off-hand, and then reverse direction of movement so that it can act as counterweight as for forehand.
Concentrate on lifting ball over net.